PROJECT SUMMARY/ABSTRACT The U.S. is experiencing a crisis on opioid use with escalating numbers of opioid overdose deaths in recent years. Sex differences exist in multiple phases of opioid dependence (OD) including acquisition, escalation, addiction, withdrawal, relapse, and treatment response. Overall, men are more likely to become dependent than women, although women tend to progress and develop medical or social consequences faster, have more difficulty discontinuing use, and are more vulnerable to relapse. However, the biological and genetic factors for these differences are poorly understood and have been understudied. We will capitalize on our existing large- scale genome-wide association study (GWAS) data, which were derived from our substance dependence (SD) cohort (N>10,000), including African and European American males and females. This SD cohort has been evaluated by the comprehensive instrument: Semi-Structured Assessment of Drug Dependence and Alcoholism (SSADDA). Here we propose to analyze the X-chromosome using the genetic data for this SD cohort. The X-chromosome data have already been collected, but have not been included in our published GWAS due to technological challenges. As advancement in biotechnology and analytical development, we are now able to analyze the X-chromosome data. We aim to identify sex-specific risk variants, and test association between OD and X-linked genetic variants for sex-specific OD patients, to characterize molecular pathways and genetic functionality, and to dissect genetic architecture between OD men and OD women. Through this study we will gain the knowledge to identify X-linked genetic variants that may assist in developing treatment strategies uniquely for men and women OD individuals to be applied in clinical treatment as remedies for the opioid crisis in the US. By completion of this study our research will aid in addressing the objectives in the three goals of the NIH strategic plan for women's health research: We will contribute to increasing sex differences research in basic science studies, incorporate sex-specific findings in our design, and contribute to sex-specific genetic profiling in personalized prevention, diagnostics and potentially treatment strategy for OD men and women.